The long-term goal is to define the mechanisms of action of the neuropeptide hormones which control vertebrate reproductive behaviors. The current proposed research represents a novel approach to the long-standing problem of understanding the precise role of sexually dimorphic neural structures. There is strong evidence for structural sexual dimorphism in the brains of vertebrates as well as behavioral sexual dimorphism. What is not known is the link between these two observations. Since the neuropeptide arginine vasotocin (AVT) is a potent activator of behavior, examining the sexual differentiation of this neuroendocrine system may provide this missing functional link between anatomical structure and behavior. The specific goals of the proposed research are (1) to determine whether gonadal steroid hormones influence the concentration of AVT in specific brain areas of adults, (2) to determine whether the presence of the gonadal steroid hormones during development is required for differentiation of the central nervous system AVT pathways, and (3) to determine whether experimental changes in sex steroid levels alter the distribution or density of AVT receptors. Concentrations of peptide hormone in specific brain areas will be measured using the combined techniques of microdissection and radioimmunoassay of brain tissue. Brain receptors for AVT will be localized and quantified using in vitro autoradiography which is uniquely suited to study of receptors in small neural regions. Correlative studies of steroid-neuropeptide interactions and the use of the extirpation-replacement experimental paradigm in animals of various ages, will determine the functional relationships between these hormones in sexual differentiation. The specific animal to be used in the amphibian Rana catesbeiana. Recent evidence demonstrates that the same steroid and neuropeptide hormones control reproduction in both mammals and amphibians. However, the simplified nervous system of amphibians makes this class of vertebrates uniquely suited to studies of the neural substrates of behavior. These studies will yield insights into basic mechanisms underlying the interaction of steroid hormones and central nervous system peptide hormones. These findings will have broad applicability. Steroids and neuropeptides also interact within the brain to control cardiovascular function and memory processes, for example.